There's an easy answer to that...don't use IoT devices. I recently rebuilt my HA setup into a VM running on Proxmox, added a Zoos USB to ZWave dongle and then replaced every device that needed a network connection with a Z-Wave device. I have nothing left that needs, or can even connect to, the Internet and all of my routines / automations are fully local. I can turn my Router off and the only thing it will impact is remote access and voice control.